Abstract
We report a small-angle neutron scattering study of the helical and skyrmion lattice order in single-crystal ${\mathrm{Cu}}_{2}{\mathrm{OSeO}}_{3}$ under quasihydrostatic helium gas pressures up to 5 kbar. By using helium gas as the pressure-transmitting medium (PTM) we ensure pressure application with improved hydrostaticity at cryogenic temperatures compared with previous reports where liquid PTMs were used. For 5-kbar He gas pressure we observe modest changes of the ambient pressure phase diagram; the critical temperature ${T}_{c}$ changes by $+2.8$(2)%, while in the low-$T$ limit the helical propagation vector $|q|$ changes by $\ensuremath{-}0.5(2)%$, the lower critical field ${H}_{c1}$ changes by $+2.5$(1.0)%, and the upper critical field ${H}_{c2}$ remains unchanged within uncertainty. The skyrmion phase also changes little under pressure; its largest $T$ extent varies from ${T}_{c}\ensuremath{-}2.5(5)$ K at ambient pressure to ${T}_{c}\ensuremath{-}3.0(5)$ K at 5 kbar, and its location in the phase diagram follows the pressure-driven shift of ${T}_{c}$. The weak pressure dependences of the critical magnetic fields and skyrmion phase contrast strongly with much stronger pressure-driven changes reported from previous quasihydrostatic pressure studies. Taking into account the present results and those of other uniaxial pressure data, we suggest that the results of previous quasihydrostatic pressure studies were influenced by inadvertent directional stress pressure components. Overall, our study represents a high-pressure study of the chiral magnetism in ${\mathrm{Cu}}_{2}{\mathrm{OSeO}}_{3}$ under the most hydrostatic high-pressure conditions to date and serves also as a salient reminder of the sensitivity of chiral magnets to deviations from hydrostaticity in quasihydrostatic high-pressure studies.
Highlights
Skyrmions are topologically protected vortexlike quasiparticles that form a hexagonal lattice in a few chiral cubic magnets under well-defined conditions of temperature T and applied magnetic field μ0H [1,2,3,4,5]
The skyrmion phase changes little under pressure; its largest T extent varies from Tc − 2.5(5) K at ambient pressure to Tc − 3.0(5) K at 5 kbar, and its location in the phase diagram follows the pressure-driven shift of Tc
At a fixed temperature and magnetic field of 58.25(10) K and μ0H [11 ̄0] = 21 mT, an in situ first-order pressure-driven transition between skyrmion lattice (SkL) and paramagnetic phases was demonstrated, indicating pressure as a third thermodynamic variable of the phase diagram of Cu2OSeO3 that can be considered for exploitation in order to achieve skyrmions at higher T s in this archetypal insulating skyrmion host material
Summary
Skyrmions are topologically protected vortexlike quasiparticles that form a hexagonal lattice in a few chiral cubic magnets under well-defined conditions of temperature T and applied magnetic field μ0H [1,2,3,4,5]. First observed in 2009 from small-angle neutron scattering (SANS) studies of the B20 chiral magnet MnSi (space group P213) [1], skyrmion lattices were observed in other P213 magnets such as FeGe [4], Fe1−xCoxSi [3], and Cu2OSeO3 [2] and, in Co-Zn-Mn alloys which crystallize in a different chiral cubic space group, P4132/P4332 [6,7,8]. Common among these chiral cubic magnets is that their equilibrium skyrmion phases are stable over a small parameter space, typically just a few Kelvin wide directly below the magnetic ordering. Finding ways to enhance the skyrmion phase stability over a broader parameter space could improve the perspective for skyrmion-based applications
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